North Korea has blamed South Korea and the United States for cyber attacks that temporarily shut down websites at a time of heightened tensions over the North's nuclear programme.

Experts believe it could take months to determine what happened and one analyst suggested more likely culprits: hackers in China.

Internet access in Pyongyang was intermittent on Wednesday and yesterday and Loxley Pacific Co, the broadband internet provider for North Korea, said it was investigating an online attack that took down Pyongyang servers. A spokesman for the Bangkok-based company said today that it was not clear where the attack originated.

North Korea's official Korean Central News Agency blamed the shutdown on the US and South Korea, accusing the allies of expanding an aggressive stance against Pyongyang into cyberspace with "intensive and persistent virus attacks".

South Korea denied the allegation and the US military declined to comment.

Loxley Pacific, which has provided broadband internet service in North Korea through a joint venture with the government since 2010, said the internet was back to normal today. Journalists in Pyongyang also were able to access the internet again after two days of disruptions. Most North Koreans do not have access to the internet, which remains restricted to a select group.

The cyber attack accusation comes amid a torrent of North Korean criticism against the US and South Korea for holding routine joint military drills that Pyongyang considers preparations for an invasion.

North Korea is also incensed by UN sanctions punishing Pyongyang for testing a nuclear device that it claims to need as a defence against US aggression. Increasingly, many nations see cyberspace as a new front for warfare. China and the US have accused one another of state-backed cyber spying.

South Korean security experts questioned North Korea's quick blame of Washington and Seoul because it can take months to trace the source of a cyber attack and hackers can easily disguise their locations. Individual hackers in China, where information about North Korea's cyberspace and computer software is more widely available than in the US and South Korea, are more likely to blame in this case, said Lim Jong-in, dean of Korea University's Graduate School of Information Security in Seoul.

"There are many Chinese internet users who have expressed their hatred of North Korea these days. I think it's more likely that some of them launched cyber attacks on North Korean websites," said Lim. "Many in China know much more about North Korea's IT environments."